Ion Depletion Microenvironments Mapped at Active Electrochemical Interfaces with Operando Freezing Cryo-Electron Microscopy

Interfacial structural and chemical evolution underpins safety, energy density, and lifetime in batteries and other electrochemical systems. During lithium electrodeposition, local nonequilibrium conditions can arise that promote heterogeneous lithium morphologies but are challenging to directly study, particularly at the nanoscale. Here we map chemical microenvironments at the active copper/electrolyte interface during lithium electrodeposition, presenting operando freezing cryogenic electron microscopy (cryo-EM), a new method, to lock in structures arising in coin cells. We find local ion depletion is correlated with lithium whiskers but not planar lithium, and we hypothesize that depletion stems from root-growing whiskers consuming ions at the growth interface while also restricting ion transport through local electrolyte. This can allow dangerous lithium morphologies to propagate, even in concentrated electrolytes, as ion depletion favors dendritic growth. Operando freezing cryo-EM thus reveals local microenvironments at active electrochemical interfaces to enable direct investigation of site-specific, nonequilibrium conditions that arise during operation of energy devices.

reconfirm that modified cells show comparable electrochemistry to conventional cells as a standard check for this technique.

Operando Plunge Freezing
Cells were cycled in an operando plunge freezer consisting of a CHI 760E electrochemical workstation, modified Thermo Fisher Vitrobot Mark IV, and custom-built operando freezing sample holder.Control of the workstation was integrated with control of the plunge freezer via a LabJack data acquisition system and Python.In order to trigger plunge freezing at a particular time point in Li electrodeposition, the actuator speed was measured and calibrated for by recording 10 instances of plunging at 240 fps and taking the average amount of time between when plunging was triggered and when the sample was fully submerged in the cryogen (1.51 s).The precision of triggering plunging based on a calibrated test time was determined by taking the standard deviation of these times (0.09 s).The effective bulk freezing rate was also measured by operando freezing a modified Li-Cu cell (prepared as described above) during linear sweep voltammetry from 3 V to 1 V at a rate of 0.1 V s −1 and taking the time for the output current to decay to 0, indicating that ions and electrolyte throughout the cell were immobilized.In addition, as the small TEM grid current collector was expected to cool much faster than the bulk cell, the sample freezing rate was validated using cryo-TEM SAED to check that the electrolyte was amorphous after plunge freezing (Fig. S3), as is standard in cryo-EM of liquids.
For Li electrodeposition studies, the environmental chamber of the Vitrobot was set to 25°C and ambient relative humidity (~40%).Current was applied at either 0.5 mA cm −2 , 2.5 mA cm −2 , or 10 mA cm −2 -as the combined surface area of the Cu mesh and TEM grid was larger than that of the Li metal punch, the Li metal surface area was used in calculating current density.Cells were plunge frozen 2, 5, and 10 min into current application (calibrating for speed of the plunge actuator as described above), with current flowing continuously throughout plunging into liquid nitrogen (LN2).LN2 was selected as the cryogen due to its non-flammable nature; flammable cryogens such as ethane were determined to be hazardous when plunging electrified samples.After the cell was fully submerged in LN2, still held by the custom sample holder, the electrochemical workstation was powered off and the cell was released from the sample holder into the cryogen dewar by pushing it out with cooled plastic tweezers.Note that as characterization was destructive, each combination of current density and time consisted of a new coin cell.
Once the cell was loose in the cryogen dewar, the dewar was gently moved from the Vitrobot platform to the lab bench.Using cooled plastic tweezers, the cell was manipulated to sit with the window facing up in the bottom of the dewar.Long tweezers were used to hold the top of a cooled nail and position its tip on the window, just to the side of TEM grid rather than directly over it to avoid damage.A hammer was used to quickly tap the nail and break the window.Cooled fine-tip tweezers were then used to extract the grid and transfer it to a cryo-EM sample box.From here, cryo-transfer to the S/TEM proceeded as normal, closing the box under LN2 and moving it quickly to the cryo-transfer station of a side-entry cryo-TEM holder or to a cryo-storage dewar or dry shipper for transport to another facility.

Cryo-EM Characterization
Characterization of the Li-Cu half cell without any current application was performed on a Thermo Fisher Scientific Spectra 200 S/TEM, operated at 200 kV, with a Gatan 626 cryotransfer holder maintained at temperatures below -160°C.All remaining samples were characterized using a Thermo Fisher Scientific Talos F200X G2 S/TEM, operated at 200 kV, with a Gatan Elsa cryo-transfer holder maintained at temperatures below -160°C.Cryo-STEM imaging and EDS was performed at spot size 6 with a 98 mm camera length and semiconvergence angle of 10.50 mrad.EDS maps were acquired using Super-X EDS detection, which leverages four silicon drift detectors positioned symmetrically around the sample to achieve high-efficiency X-ray collection while reducing potential shadowing effects.The EDS dwell time was 2 μs and yielded screen currents around 20 pA.

SEM
Samples for SEM were prepared by assembling coin cells for Li electrodeposition as described above, but using as-is stainless steel coin cell cans (without windows) and pressing for 5 s to seal.No Cu mesh besides the TEM grid was used, as the whole can was already conductive; this did not affect the current density compared to the modified cells as that was still limited by the area of the Li punch.Cells were rested for 2 hrs, after which Li electrodeposition was performed on a Maccor Series 4000 held at 25°C at 0.5, 2.5, or 10 mA cm −2 for 10 min.Cells were then removed from the Maccor, brought into an Ar glovebox, and disassembled.The TEM grids were removed from the disassembled cells and gently rinsed in dimethyl carbonate for ~10 min each to remove residual electrolyte.The samples were left to dry under Ar for 2 hrs before mounting onto an SEM stub inside the glovebox using double-sided carbon tape.The stub was then placed in the antechamber of the glovebox and vacuum pumped for an additional 10 min to ensure samples were fully dry.Finally, the stub was brought back into the glovebox, sealed under Ar, and wrapped in Parafilm before removing for transfer to the SEM to minimize air exposure.SEM images were acquired via the secondary electron (SE) detector on a Hitachi S-4800 Scanning Electron Microscope.Images were captured at an accelerating voltage of 3 kV, an emission current of 7 μA, and at working distances between 3.6 and 3.9 mm.

EDS Data Analysis
EDS quantification was conducted in Velox to yield atomic fractions across line scans.Quantification was improved using a pixel averaging pre-filter (typically 5 or 10px) to compensate for low signal-to-noise in EDS acquisitions due to low electron doses.Increasing the number of pixels averaged prior to quantification increased the signal-to-noise and brought the calculated P atomic fraction closer to the theoretical value generated by the finite-element model, at the expense of spatial resolution.However, pixel averaging did not change the qualitative shape of the data (e.g., whether the profile was relatively flat or showed a depleted region); thus, a 5-10px average filter was generally sufficient to improve the quantification while maintaining spatial resolution, but did not change the presence or absence of the depleted region.
After quantification, the P atomic fraction was normalized based on the summed C, P, O, and F atomic fractions, such that the Cu atomic fraction did not influence the final plotted profiles-this ensured that spatial variation in the plotted P atomic fractions reflected variations in the electrolyte composition, rather than variations in the amount of signal coming from the Cu versus the electrolyte.EDS line scans showing unusual artifacts or contamination (e.g., O-rich features) were excluded from analysis.The remaining scans were calibrated as illustrated in Fig. S5 to align them such that drop-off of the Cu signal (from the edge of the grid bar) occurred at x=0.Calibrated scans were labelled as either showing a relatively flat P composition or an increasing P composition gradient for the statistical analyses shown in Fig. S6.For width measurements of the depleted regions, the two least noisy scans were taken from each of the time points for deposition at 10 mA cm −2 and the widths of their depleted regions were averaged.Blue bars and wedges represent regions for which a flat P composition is observed, while red bars and wedges represent regions for which an increasing P composition gradient is observed.(a) Summary of behavior observed for all line scans at 0.5 mA/cm 2 , 2.5 mA/cm 2 , and 10 mA/cm 2 .Regions scanned in samples deposited at 0.5 mA/cm 2 generally show a flat P composition, while a P composition gradient is increasingly prevalent for samples deposited at higher current densities.Pie charts showing all regions scanned for samples deposited at (b) 0.5 mA/cm 2 or (c) 10 mA/cm 2 show a P composition gradient is rare at low current density but observed in the majority of line scans at high current density.,c) 200 min of deposition at 0.5 mA/cm 2 (1.7 mAh/cm 2 ); (d) 10 min of deposition at 2.5 mA/cm 2 (0.42 mAh/cm 2 ); and (e,f) 10 min of deposition at 10 mA/cm 2 (1.7 mAh/cm 2 ).Deposition at 0.5 mA/cm 2 generally gives rise to uniform Li grains with a minimal amount of Li moss and negligible growth into the pore of the Cu grid, while 2.5 mA/cm 2 and 10 mA/cm 2 show Li whiskers and extensive growth into the pore.Deposition at 10 mA/cm 2 s is also significantly less uniform across the grid (f) than at 0.5 mA/cm 2 s (c), pointing to the presence of heterogeneous microenvironments.

Finite-Element Model Details
The finite-element model is developed in COMSOL Multiphysics 6.2.The domain includes a uniform Li-ion flux from an "Electrode Surface" with the average current density being specified at the top-most boundary (located at the top of the separator domain in Fig. S11), a porous separator domain, and two electrolyte domains with reactive boundaries.In the electrolyte domain, the reactive boundaries are set to the reference potential.Fig. S12a illustrates the reactive boundaries.All other boundaries are set to noflux due to symmetry.Within the electrolyte domain, Li-ion species conservation is resolved using concentrated solution theory of a bipolar salt, which is common in standard pseudo-2D battery models.
The governing equation for salt conservation can be expressed mathematically as Where   is the electrolyte volume fraction,  e is the salt concentration,   is the Li-ion diffusion coefficient,   is the Bruggeman coefficient,   is the ionic current in the electrolyte,  + 0 is the Li-ion transport coefficient,   is the electrolyte ionic conductivity, Φ  is the electrolyte electrostatic potential,  is the universal gas constant,  is temperature,  is Faraday's constant, and ) is the thermodynamic factor.The four transport related parameters in the electrolyte are taken for Gen2 electrolytes and can be represented as a function of salt concentration and temperature as The "effective" parameters to account for porous media effects (i.e.,   , and   ) are only used in the separator region.In all other regions   = 1.At the Cu reactive boundaries, the Li-ions are removed from the simulation via a Butler-Volmer relationship, which can be expressed as where  is the Li-ion flux at the reactive surface,  0 is the exchange current density,  is the cathodic transference number, Φ  is the solid-phase potential (set to reference), and   is the open-circuit potential of the Li-plating reaction (set to 0 [V] vs. Li/Li+).Table S1 documents the model parameters.Importantly, the model described here assumes that the plated Li does not strongly influence the electrolyte volume fraction or overall volume (i.e., deposited Li leaves the model domain).As a back-ofthe-hand calculation, if the Li is assumed to plate uniformly on the reactive surface, and that the Li deposits are planar and non-porous, the deposited Li would grow approximately 2 μm into the domain on the depositing surfaces after demanding 10 mA cm -2 for 10 min.With a nominal Cu mesh size of 152 μm and a test-cell size of 54 μm, the domain shrinkage due to plated Li is relatively small.Fig. S12a illustrates the model domain with dimensions described by Table S1.Fig. S12b illustrates the Li-ion inlet surface (red) and reactive deposition surfaces (blue).Any surface not colored is set to a symmetry (or no-flux) boundary condition.Fig. S12c-d illustrate a sample of the model predicted results generated for the 10 mA cm -2 condition after 5 min.As shown, the electrolyte current density is uniform at the top of the separator and Li plating reactions occur favorably at the top of the Cu section with progressively less deposition reactions occurring further down the domain.Fig. S12e-f indicate how the 3-D salt concentration distribution is averaged/projected in the Test cell (Fig. S12e) onto a 2D surface (Fig. S12f).Additionally, Fig. S12f indicates that the averaged salt concentration provided in the main manuscript is reported along the center symmetry line.Fig. S13 shows the concentration along this line for 0.5 mA cm -2 (Fig. S13a) or 10 mA cm -2 (Fig. S13b) at 2, 5, and 10 min, with y-axes zoomed to show the minor concentration profile predicted, which is negligible in comparison with experimental results.The modelled and experimental atomic fractions were then normalized by the atomic fraction 1 μm into the electrolyte (for EDS measurements, this was determined by taking an average of the last 5 data points before 1 μm), which represented the "plateau" value for all samples.

Figure S1 :
Figure S1: Modified coin cell, operando freezing, and cold sample removal process.(a) Diagram of modified coin cell, with close-up of modified can (b) and photo of assembled cell (c).(d) Workflow for cold sample removal: Modified cell is plunge frozen using the operando plunge freezer with electrochemical sample holder, the window is broken while submerged under cryogen, and the working electrode is removed for characterization.

Figure S2 :
Figure S2: (a) Li electrodeposition onto Cu TEM grids at 0.5 mA/cm 2 in a modified coin cell (blue) shows a similar voltage profile as in a conventional cell (black).(b) Voltage profile during operandofreezing 30 min into Li electrodeposition at 0.5 mA/cm 2 using modified coin cell and custom electrochemical holder for plunge freezer (red), compared to electrodeposition in conventional (black) or modified (blue) cells using standard Maccor cycler.

Figure S3 :
Figure S3: Method validation.(a) Cryo-STEM EDS from region of bulk Gen2 electrolyte shows expected elemental composition, without notable contamination from air or cryogen.(b) Cryo-TEM SAED of sample operando frozen during Li deposition onto Cu does not show evidence of electrolyte crystallization during freezing.(b) Operando freezing during linear sweep voltammetry of a Li-Cu half cell shows it takes ~3 s for ion transport to be sufficiently kinetically halted such that the output current decays to 0.

Figure S4 :
Figure S4: Example cryo-STEM HAADF images used for EDS mapping and corresponding EDS maps and line scans from samples operando frozen 10 min into Li deposition at (a) 0.5 mA/cm 2 or (b) 10 mA/cm 2 .EDS maps are shown with 10px-average post-processing filter to reduce noise.No structural features besides the edge of the Cu grid bar and uniform layer of vitrified electrolyte are apparent in the 0.5 mA/cm 2 case, consistent with observations of uniform planar Li with minimal growth into pores at this current density.At the higher current density, contrast variation in the electrolyte region is consistent with nonuniform Li growth into the pore, leading to regions with lower Z contrast in HAADF imaging.

Figure S5 :
Figure S5: Illustration of calibration of x = 0 position in cryo-STEM EDS data, for a sample operando frozen at 2 min of Li electrodeposition onto a Cu TEM grid at 10 mA/cm 2 .Step 1: Raw EDS data from line scan from Cu grid bar into electrolyte is used to plot (a) the total atomic fraction of Cu (considering all mapped elements) and (b) the P atomic fraction considering only detectable electrolyte elements C, O, F, and P (excluding Cu).The Cu signal shows a steep drop-off, as in (a), corresponding to the end of the grid bar.Step 2: The point of Cu drop-off is used to correct the distance axis of the EDS data, such that x = 0 corresponds to the edge of the Cu grid bar for all samples.(c) and (d) show the Cu and P data, respectively, re-plotted after this correction, with the width of the P-depleted region marked by the black arrow and dashed line in (d).

Figure S6 :
Figure S6: Statistical breakdown of features observed in EDS line scans of operando frozen samples.Blue bars and wedges represent regions for which a flat P composition is observed, while red bars and wedges represent regions for which an increasing P composition gradient is observed.(a) Summary of behavior observed for all line scans at 0.5 mA/cm 2 , 2.5 mA/cm 2 , and 10 mA/cm 2 .Regions scanned in samples deposited at 0.5 mA/cm 2 generally show a flat P composition, while a P composition gradient is increasingly prevalent for samples deposited at higher current densities.Pie charts showing all regions scanned for samples deposited at (b) 0.5 mA/cm 2 or (c) 10 mA/cm 2 show a P composition gradient is rare at low current density but observed in the majority of line scans at high current density.

Figure S7 :
Figure S7: Cryo-STEM EDS phosphorus atomic fraction profiles scanning from Cu current collector to electrolyte for samples operando frozen at (a) 10 min of Li electrodeposition at 0.5 mA/cm 2 or (b) 2 minof Li electrodeposition at 2.5 mA/cm 2 , both to the same capacity of 0.083 mAh/cm 2 , shows the appearance of a P atomic fraction gradient is correlated with current density, rather than capacity.

Figure S8 :
Figure S8: Cryo-STEM EDS phosphorus atomic fraction profile scanning from Cu current collector to electrolyte for sample frozen without any applied current.

Figure S9 :
Figure S9: SEM images of Li morphology on Cu grids after (a) 10 min of deposition at 0.5 mA/cm 2 (0.083 mAh/cm 2 ); (b,c) 200 min of deposition at 0.5 mA/cm 2 (1.7 mAh/cm 2 ); (d) 10 min of deposition at 2.5 mA/cm 2 (0.42 mAh/cm 2 ); and (e,f) 10 min of deposition at 10 mA/cm 2 (1.7 mAh/cm 2 ).Deposition at 0.5 mA/cm 2 generally gives rise to uniform Li grains with a minimal amount of Li moss and negligible growth into the pore of the Cu grid, while 2.5 mA/cm 2 and 10 mA/cm 2 show Li whiskers and extensive growth into the pore.Deposition at 10 mA/cm 2 s is also significantly less uniform across the grid (f) than at 0.5 mA/cm 2 s (c), pointing to the presence of heterogeneous microenvironments.

Figure S11 :
Figure S11: Geometry of finite-element model represents ¼ of the active Li deposition area, reduced by symmetry, and reflects structure of the modified cell, with the Cu TEM grid over a non-reactive surface (cell window), followed by a Cu mesh which provides a conductive pathway across the window, and polymer separator.

Figure S12 :
Figure S12: a) Finite-element model domain descripting (representing ¼ of the active Li deposition area), b) inlet and Li-deposition reactive surfaces, c) sample model results of electrolyte current density during the 10 mA cm -2 deposition rate after 5 min, d) side-view of electrolyte current density during the 10 mA cm -2 deposition after 5 min, e) sample salt concentration predicted by the model in the test cell, f) averaged salt concentration within the test cell projected onto the bottom surface with an indicator for the cell center line where results were extracted.

Figure S13 :
Figure S13: Averaged salt concentration predicted by the finite-element model along the center symmetry line of the Cu pore geometry, for Li deposition at (a) 0.5 mA cm -2 or (b) 10 mA cm -2 , at 2, 5, or, 10 min.Note the y-axes scales differ between (a) and (b) and are zoomed to show the minor concentration profile predicted by the model, which is negligible in comparison to experimental results.

Table S1 :
Constant physics-based model parameters.